The application and incorporation of crop residue and animal manure to soils has been recognized by modern agriculture as providing a number of benefits that enhance crop yields. With today's specialized agricultural enterprises, crop ground is seldom allowed to lie fallow or be placed in green manure crops. Livestock rearing has also become specialized which has resulted in concentration of animal population into confinement operations that may have little, if any, additional crop ground for manure disposal. How the manure is handled not only impacts the profitability of the livestock operation and the environment, but also impacts the overall health of the livestock population.
The concentration of livestock has resulted in the environmental regulation of these enterprises and their waste disposal practices. Currently, many states regulate the amount of waste that can be disposed of upon an area of land, as well as when it may be applied. In fact, where such facilities are near populations, odor control is a factor.
Most manures are at least 80% water. Since the fertilizer value is generally associated with the solids fraction, a number of methods adapted from municipal water treatment, such as screening, digesters, and settling ponds have been used to concentrate the solid fraction. The result is a liquid (supernatant) and a sludge. The former can easily be handled with conventional pumping systems while the latter must be trucked and spread.
The stabilization of organic material has been historically performed using both aerobic and anaerobic fermentation techniques. These techniques include, but are not limited to: